Abstract
Key messageHygroscopicity is a crucial element of bark water storage and can reach >60% of water holding capacity of bark depending on tree speciesBark forms the outer layer of woody plants, and it is directly exposed to wetting during rainfall and reacts to changes in relative humidity, i.e., it may exchange water with the atmosphere through absorption and desorption of water vapor. A current paradigm of bark hydrology suggests that the maximum water storage of bark empties between precipitation events and is principally controlled by bark thickness and roughness. We hypothesize that (1) the ability of bark to absorb water vapor during non-rainfall periods (i.e., hygroscopicity) leads to partial saturation of bark tissues during dry periods that may alter the rate of bark saturation during rainfall, and (2) the degree of bark saturation through hygroscopic water is a function of internal bark structure, including porosity and density, that varies among species. To address these questions, we conducted laboratory experiments to measure interspecific differences in bark physical structure as it relates to water storage mechanisms among common tree species (hickory (Carya spp.), oak (Quercus spp.), sweetgum (Liquidambar styraciflua), and loblolly pine (Pinus taeda)) in the southeastern United States. Furthermore, we considered how these properties changed across total bark, outer bark, and inner bark. We found a distinct difference between hickory and oak, whereby hickory had 5.6% lower specific density, 31.1% higher bulk density, and 22.4% lower total porosity of outer bark resulting in higher hygroscopicity compared to oaks. For all species, hygroscopicity increased linearly with bulk density (R2 = 0.65–0.81) and decreased linearly with total porosity (R2 = 0.64–0.88). Overall, bark hygroscopicity may constitute an average of 30% of total bark water storage capacity. Therefore, in humid climates like those of the southeastern USA, the proportion of bark that remains saturated during non-storm conditions should not be considered negligible.
Highlights
Bark constitutes the outermost layer of stems, branches, and roots of trees and shrubs, and plays very different functions during the life cycle of woody plants
We hypothesize that (1) the ability of bark to absorb water vapor during nonrainfall periods leads to partial saturation of bark tissues during dry periods that may alter the rate of bark saturation during rainfall, (2) the degree of bark saturation through hygroscopic water is a function of internal bark structure, including porosity and density, that varies among species, and 3) bark hygroscopicity represents a non-negligible proportion of bark water storage capacity in the species of study
Among the broadleaved tree species considered in this study, they can be divided into two groups based on the ratio of the thickness of outer bark to total bark: (1) species whose inner bark contributes to the majority of total bark thickness, and (2) species whose outer bark contributes to the majority of total bark thickness (Table 2)
Summary
Bark constitutes the outermost layer of stems, branches, and roots of trees and shrubs, and plays very different functions during the life cycle of woody plants. It separates underlying living tissues from the atmosphere and in case of the bark. Many studies emphasize the role of bark properties in the protection of trees during fire events (Pinard and Huffman 1997; Bauer et al 2010; Staver et al 2020). Apart from the physical protection of trees, bark has important physiological functions, like wound repair, photosynthesis, and storage of organic compounds (Dossa et al 2018). Since bark can absorb water from the air and store liquid water from rainfall, stemflow, dew, and fog, bark helps to protect trees from desiccation as well (Romero 2014)
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